In a broad sense, “pitch” is any “sticky” substance found in the pulp and papermaking process. These substances create quality and efficiency problems in the paper making process when they deposit on paper making equipment. More specifically “pitch” refers to any soluble organic matter originating from the extracts of wood during pulping which includes fatty acids and esters, resin acids, and sterols. Pitch may also include process additives such as defoamers, sizing agents, and coatings, as well as inorganic components like calcium carbonate, silica, clay, magnesium and titanium. Pitch is released from wood during chemical and mechanical pulping.
Fine particles of pitch that remain well-dispersed do not create a deposition problem. However, there is a tendency for the hydrophobic pitch particles to agglomerate at the air-water interface. These pitch particles form deposits due to their decreased solubility as they move downstream (through cooler temperatures and lower pH) and from the increased turbulence of the process. These pitch particles aggregate to form tacky threads or balls which then deposit on pulp and paper making equipment including chest walls, screens, paper machine headbox, wires, felts and dryers. This can also lead to sheet holes and breaks in paper resulting in increased downtime and/or lower quality paper. Such pitch deposition can be also be a problem in recycled or secondary fiber processes where synthetic organic polymeric organo-soluble pitch contaminants called “stickies”, such as hot melts, polyethylene films, latexes, pressure sensitive adhesives, and waxes agglomerate and form deposits on papermaking equipment.
Typically used pitch control agents include nonionic surfactants, especially nonylphenol ethoxylates, talc and alum. Nonionic surfactants, which contain both a hydrophobic and hydrophilic segment, decrease the surface tension, provide wetting, detergency and dispersancy. However, nonylphenol ethoxylates have a high foaming tendency which may result in higher defoamer usage, thus depositing more pitch.
Talc is also used to control pitch deposition by adsorbing colloidal pitch onto the talc particles so it is retained in the sheet. The disadvantage of using talc is that it tends to deposit further downstream especially in paper machine felts and cause felt plugging. In acid paper making systems, alum or aluminum sulfate is used to decrease the tackiness of the pitch by cationic fixation to pulp. When using alum, pH control is critical to prevent deposition of alum.
Inorganic dispersants like sodium polyacrylate, and similar compounds, sequester or chelate the inorganic portion of the pitch particle, but leave the organic part free to interfere with low foam tolerant processes as well as increase defoamer usage.
Current technology in pitch or stickies control involves the use of detackifiers, which are hydrophilic polymers, to treat the hydrophobic pitch or stickies. The hydrophilic polymers are adsorbed onto the hydrophobic pitch or stickies particles, thereby converting the hydrophobic pitch or stickies particles to hydrophilic particles. The pitch or stickies particles, which are now hydrophilic, are likely stay with the pulp or paper and less likely to deposit on any kind of machinery, felt or wire surfaces.
Some known detackifiers are polyvinyl alcohol (PVA)1, methylcellulose2, and polyvinylacetate/ethyleneoxide3. 1 See U.S. Pat. No. 4,871,424.2 See U.S. Pat. No. 4,846,933.3 See U.S. Pat. No. 5,266,166.